The present invention relates to a photothermographic material. In particular, the present invention relates to a photothermographic material for scanners, image setters and so forth, which is particularly suitable for photographic art. More precisely, the present invention relates to a photothermographic material that shows low humidity dependency during development for developed character line width and can secure high image density (Dmax) even in a low humidity environment.
There are known many photosensitive materials having a photosensitive layer on a support, with which image formation is attained by imagewise light exposure. Those materials include those utilizing a technique of forming images by heat treatment as systems that can contribute to the environmental protection and simplify image-forming means.
In recent years, reduction of amount of waste processing solutions is strongly desired in the field of photographic art from the standpoints of environmental protection and space savings. Therefore, development of techniques relating to photothermographic materials for photographic art is required, which materials enable efficient exposure by a laser scanner or laser image setter and formation of clear black images having high resolution and sharpness. Such photothermographic materials can provide users with simpler and non-polluting heat development processing systems that eliminate the use of solution-type processing chemicals.
Methods for forming images by heat development are described in, for example, U.S. Pat. Nos. 3,152,904 and 3,457,075 and D. Klosterboer, xe2x80x9cThermally Processed Silver Systems Axe2x80x9d, Imaging Processes and Materials, Neblette, 8th ed., compiled by J. Sturge, V. Walworth and A. Shepp, Chapter 9, p.279, (1989). Such photothermographic materials comprise a reducible non-photosensitive silver salt (e.g., silver salt of an organic acid), a photocatalyst (e.g., silver halide) in a catalytically active amount and a reducing agent for silver, which are usually dispersed in an organic binder matrix. While the photosensitive materials are stable at an ordinary temperature, when they are heated to a high temperature (e.g., 80xc2x0 C. or higher) after light exposure, silver is produced through an oxidation-reduction reaction between the reducible silver salt (which functions as an oxidizing agent) and the reducing agent. The oxidation-reduction reaction is accelerated by catalytic action of a latent image generated upon exposure. The silver produced from the reaction of the reducible silver salt in the exposed areas shows black color and provides contrast with respect to the non-exposed areas, and thus images are formed.
In many of conventionally known photothermographic materials, an image-forming layer is formed by coating a coating solution using an organic solvent such as toluene, methyl ethyl ketone (MEK) and methanol as a solvent. However, not only use of an organic solvent as a solvent adversely affect human bodies during the production process, but also it is disadvantageous in view of cost because it requires process steps for recovery of the solvent and so forth.
Accordingly, methods of forming an image-forming layer by coating a coating solution using water as a solvent have been proposed. For example, Japanese Patent Laid-open Publication (Kokai, hereinafter referred to as JP-A) 49-52626, JP-A-53-116144 and so forth disclose image-forming layers utilizing gelatin as a binder, and JP-A-50-151138 discloses an image-forming layer utilizing polyvinyl alcohol as a binder. Furthermore, JP-A-60-61747 discloses an image-forming layer utilizing gelatin and polyvinyl alcohol in combination. As another example, JP-A-58-28737 discloses an image-forming layer utilizing a water-soluble polyvinyl acetal as a binder. If these binders are used, image-forming layers can be formed by using a coating solution comprising an aqueous solvent, and therefore considerable merits can be obtained with respect to environment and cost.
However, when a polymer such as gelatin, polyvinyl alcohol or water-soluble polyacetal is used as a binder, silver tone of developed areas becomes brown or yellow, which quite differs from black color regarded as a preferred proper color, and in addition, there arises, for example, a problem that the blacking density in exposed areas is low and the density in unexposed areas is high. Thus, there can be obtained only images of which commercial value is seriously impaired. Furthermore, since such polymers show bad compatibility with the silver salt of an organic acid, there may also arise a problem that practically acceptable coatings cannot be obtained in view of coated surface quality.
European Patent Publication (hereinafter referred to as EP-A) 762,196, JP-A-9-90550 and so forth disclose that high-contrast photographic property can be obtained by incorporating Group VII or VIII metal ions or metal complex ions into photosensitive silver halide grains for use in photothermographic materials, or incorporating a hydrazine derivative into the photosensitive materials.
For use of photographic art films in the fields of newspaper printing, commercial printing and so forth, there have generally been desired systems that can provide stable images at any time. However, photothermographic materials showing such high-contrast photographic property as mentioned above, which is required for photographic art films, suffer from a problem that they show higher humidity dependency of character line width during development compared with conventional films to be treated with chemicals.
Therefore, it has been desired to provide a photothermographic material that shows low humidity dependency of character line width during development and thus is suitable for use in photographic art.
Therefore, a first object to be achieved by the present invention is to provide a photothermographic material that shows low humidity dependency of character line width during development and can secure high image density (Dmax) even in a low humidity environment, in particular, as a photothermographic material for photographic art, more specifically, a photothermographic material for scanners, image setters and so forth.
A second object to be achieved by the present invention is to provide a photothermographic material that can be prepared by coating of an aqueous system, which is advantageous for the aspects of environment and cost.
The inventors of the present invention assiduously studied in order to achieve the aforementioned objects. As a result, they found that an excellent photothermographic material that provides the desired effects could be obtained by preparing a photothermographic material so that the material could have a saturation swelling time of 60 seconds or longer in distilled water at 21xc2x0 C., and thus accomplished the present invention.
That is, the present invention provides a photothermographic material having an image-forming layer that contains at least a non-photosensitive silver salt of an organic acid, a photosensitive silver halide, a nucleating agent and a binder on a support, and a protective layer at a position remoter from the support compared with the image-forming layer, which shows a saturation swelling time of 60 seconds or longer in distilled water at 21xc2x0 C.
Preferably, the saturation swelling time is 80 seconds or longer.
Preferably, the binder of the image-forming layer comprises 50 weight % or more of latex of a polymer showing a glass transition temperature of xe2x88x9230xc2x0 C. to 40xc2x0 C.
Preferably, the protective layer has a thickness of 3 xcexcm or more.
According to another aspect of the present invention, there is provided a method for forming images, which comprises subjecting the aforementioned photothermographic material of the present invention to a heat treatment at a line speed of 140 cm/minute or higher.
According to a further aspect of the present invention, there is provided a method for forming images, which comprises exposing the aforementioned photothermographic material of the present invention for an exposure time of 10xe2x88x927 second or less.
According to a still further aspect of the present invention, there is provided a method for forming images, which comprises exposing the aforementioned photothermographic material of the present invention by using a multi-channel light source having two or more of laser heads.
According to the present invention, there can be obtained photographic properties suitable for photographic art including low development humidity dependency for line width and feasibility of securing sufficient image density (Dmax) even in a low humidity environment. Further, it enables coating with an aqueous system, which is advantageous for environment and cost.